1. Field of the Invention
The present invention relates to light emitting nitride semiconductor devices having high luminous efficacy, and light emitting apparatus and optical pickup devices employing the same.
2. Description of the Background Art
Japanese Patent Laying-Open No. 8-64870 discloses a light emitting device having a stack of gallium nitride based compound semiconductor layers, wherein an active layer is formed of a gallium nitride based compound semiconductor in which the nitrogen is partially replaced with phosphorus and/or arsenic and the active layer is doped with at least one dopant selected from the group consisting of Mg, Zn, Cd, Be, Ca, Mn, Si, Se, S, Ge and Te. In this prior art, the active layer is doped for the purpose of making the emission wavelength longer. According to the publication, the dop ant introduced into the active layer allows a different emission level to be generated in the energy band of the crystal, resulting in a longer emission wavelength. The half-width of the emission intensity spectrum relative to the emission wavelength can also be changed depending on the combination of the dopants. If, for example, Zn and Se are the dopants, an emission level as ZnSe can be established, which is different from the level produced by doping Zn or Se and also different from a simple sum of the levels by Zn and Se. The publication, however, does not disclose any specific amount of the dopant(s) introduced into the active layer. The publication only discloses a light emitting device formed on a sapphire substrate specifically. The publication also fails to note the crystallinity of each layer formed on the substrate.
Japanese Patent Laying-Open No. 10-270804 discloses a light emitting nitride semiconductor device having a light emitting layer (an active layer) with a multi quantum well structure formed of GaNAs, GaNP or GaNSb well layer/GaN barrier layer. In this publication, a sapphire (xcex1-Al2O3) substrate and a SiC substrate are examples of the substrate. The publication does not disclose any doped active layer.
The light emitting layer formed of GaNAs, GaNP or GaNSb crystal can provide smaller effective mass of electrons and holes as compared with InGaN crystal conventionally used. This suggests that a population inversion for lasing can be obtained by a small carrier density (and that a lasing threshold current value can be reduced). If, however, As is contained in the light emitting layer of nitride semiconductor, the light emitting layer can readily be separated into a higher nitrogen content region and a higher As content region. Hereinafter this phenomenon will be referred to as xe2x80x9ccomposition separationxe2x80x9d. The crystal system can further be separated into a hexagonal system of the higher nitrogen content region and the cubic system of the higher As content region. Such a separation into different crystal systems hereinafter referred to as xe2x80x9ccrystal system separationxe2x80x9d) can result in a reduced luminous efficacy due to the degraded crystallinity. Such crystal system separation can be occurred in a P- or Sb-containing light emitting nitride semiconductor layer as well as in the As-containing layer. Thus, it has been desired that such crystal system separation should be prevented for improved luminous efficacy (or emission intensity).
An object of the present invention is to provide a light emitting device having a higher luminous efficacy or emission intensity by clarifying a structure capable of enhancing the performance of the light emitting device using a light emitting layer of nitride semiconductor containing at least one of As, P and Sb.
The present inventors have found that the crystal system separation due to As, P or Sb contained in the light emitting nitride semiconductor layer can be reduced by doping the light emitting layer with an impurity of at least one element of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and Te, so that the light emitting nitride semiconductor device can have a good crystallinity and a high luminous efficacy (or emission intensity).
Thus, the present invention is directed to a light emitting nitride semiconductor device including: one of a substrate made of nitride semiconductor crystal and a substrate having a nitride semiconductor crystal film grown on a crystalline material other than the nitride semiconductor crystal; an n-type layer and a p-type layer each made of nitride semiconductor formed on the substrate; and a light emitting layer provided between the n- and p-type layers. The light emitting layer is formed of a well layer or a combination of well and barrier layers. Of the layer(s) forming the light emitting layer, at least the well layer is made of a nitride semiconductor containing an element X, N and Ga, wherein the element X is at least one selected from the group consisting of As, P and Sb.
In the nitride semiconductor forming the light emitting layer, the ratio of the number of element X atoms to the total number of the element X atoms and N atoms, is not more than 30 atomic percent. Of the layer(s) forming the light emitting layer, at least the well layer contains as an impurity at least one element selected from the group consisting of Mg, Be, Zn, Cd, C, Si, Ge, Sn, O, S, Se and Te.
In the present invention, the total content of the impurity is 1xc3x971017 to 5xc3x971020/cm3.
In the present invention, preferably, the nitride semiconductor crystal or the nitride semiconductor crystal film of the substrate, or the light emitting nitride semiconductor device has a threading dislocation density of not more than 3xc3x97107/cm2 or an etch pit density of not more than 7xc3x97107/cm2.
In the present invention, typically, the light emitting layer may be a multi-quantum well layer.
The present invention is also directed to a light emitting apparatus comprising the light emitting nitride semiconductor device as aforementioned and having an emission wavelength of 380 nm to 650 nm.
The present invention is also directed to an optical pickup apparatus including a light emission apparatus comprising the light emitting nitride semiconductor device as aforementioned and having an oscillation wavelength of 380 nm to 420 nm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.